Proton transport structural diffusion

The total electro-osmotic coefficient = Whydr + mo includes a contribution of hydrodynamic coupling (Whydr) and a molecular contribution related to the diffusion of mobile protonated complexes—namely, H3O. The relative importance, n ydr and depends on the prevailing mode of proton transport in pores. If structural diffusion of protons prevails (see Section 6.7.1), is expected to be small and Whydr- If/ ori the other hand, proton mobility is mainly due to the diffusion of protonated water clusters via the so-called "vehicle mechanism," a significant molecular contribution to n can be expected. The value of is thus closely tied to the relative contributions to proton mobility of structural diffusion and vehicle mechanism. ... 

Structural diffusion is favored by conditions that enhance the stiffness of the hydrogen-bonded network between water molecules low temperatures and low acid concentration. The decrease in water content leads to an effective increase in the concentration of acid protons, which in turn suppresses the contribution of structural diffusion, as found in aqueous acidic solutions. This agrees with the finding of an enhanced contribution of vehicular transport in PEMs at low hydration. Such an observation is also supported by recent studies of molecular mechanisms of proton transport in PEMs at minimal hydration. ... 

A more recent view of proton transport is that of Kreuer, who, compared with the Zundel-based view, describes the process on different structural scales within phase separated morphologies. The smallest scale is molecular, which involves intermolecular proton transfer and the breaking and re-forming of hydrogen bonds. When the water content becomes low, the relative population of hydrogen bonds decreases so that proton conductance diminishes in a way that the elementary mechanism becomes that of the diffusion of hydrated protons, the so-called vehicle mechanism . 

Structure diffusion (i.e., the Grotthuss mechanism) of protons in bulk water requires formation and cleavage of hydrogen bonds of water molecules in the second hydration shell of the hydrated proton (see Section 3.1) therefore, any constraint to the dynamics of the water molecules will decrease the mobility of the protons. Thus, knowledge of the state or nature of the water in the membrane is critical to understanding the mechanisms of proton transfer and transport in PEMs. 

The functional and morphological heterogeneity of a lamellar system of chloroplasts indicates that pH values in different compartments (in granal and intergranal thylakoids) differ. This type of structure makes it difficult to measure local pH values at different sites. Therefore, mathematical models taking into account the spatial structure of chloroplasts provide a tool for studying the effect of diffusion restrictions on pH distributions over the thy lakoid on the rates of electron transport, proton transport, and ATP synthesis. The rate of ATP synthesis depends on the osmotic properties of a chloroplast-incubation medium and, therefore, on topological factors. 

In this first task, each excess proton is permanently attached to a hydronium ion. This assumption prohibits stractural diffusion of the proton. However, for the purposes of the first task, namely the generation of molecular-level stmcture of the hydrated membrane and its interfaces, this approximation is adequate. For the second task, namely the generation of transport properties, this limitation is removed. Although, the classical MD simulations in task I cannot quantitatively characterize the stmctural diffusion mechanism, from the analysis of the hydration structure of the hydronium ions in these simulations the characteristics of Zundel and Eigen ion (which are necessary for structural diffusion) can be studied. 

In this work, we have approaehed the understanding of proton transport with two tasks. In the first task, deseribed above, we have sought to identify the moleeular-level stmeture of PFSA membranes and their relevant interfaees as a funetion of water content and polymer architecture. In the second task, described in this Section, we explain our efforts to model and quantify proton transport in these membranes and interfaces and their dependence on water content and polymer architecture. As in the task I, the tool employed is molecular dynamics (MD) simulation. A non-reactive algorithm is sufficient to generate the morphology of the membrane and its interfaces. It is also capable of providing some information about transport in the system such as diffusivities of water and the vehicular component of the proton diffusivity. Moreover, analysis of the hydration of hydronium ion provides indirect information about the structural component of proton diffusion, but a direct measure of the total proton diffusivity is beyond the capabilities of a non-reactive MD simulation. Therefore, in the task II, we develop and implement a reactive molecular dynamics algorithm that will lead to direct measurement of the total proton diffusivity. As the work is an active field, we report the work to date. 

Some attempts to inclnde structural diffusion exist. The mechanism of proton transport in bulk water has been studied by various molecular modeling techniques like the Car-Parinello ab initio molecnlar dynamics simnlations (CPAIMD), mixed quan-tnm and classical mechanics technique (QM/MM), E " ... 

The functional form of the triggers ate based on transition state, as determined by the quantum mechanical calculation and their numerical values are parameterized to satisfy the macroscopically determined rate constant and activation energy. Local equilibration at the end of the reaction helps in maintaining the correct heat of reaction and structure. For the vahdation of the algorithm, it has been implemented to study proton transport in bulk water. In bulk water the two components of the total diffusivity were found to be uncorrelated. 

However it turned out that the structural, chemical and dynamical details are essential for complex descriptions of long-range proton transport. These parameters appear to be distinctly different for different families of compounds, preventing proton conduction processes from being described by a single model or concept as is the case for electron transfer reactions in solutions (described within Marcus theory [23]) or hydrogen diffusion in metals (incoherent phonon assisted tunneling [24]). 

As expected, the confinement of phosphoric acid in the PBI matrix does not give rise to any relevant electroosmotic drag. Of course, the main reason is the fact that proton conductivity is dominated by structure diffusion, that is the transport of protonic charge carriers and phosphoric acid are effectively decoupled. The other reason is that protonic charge carriers are produced by self-dissociation of the proton solvent (phoshoric acid), that is the number of positively and negatively... 

Ans. During the passage of NADH through the electron transport system, protons are released to the outside of the mitochondrial membrane. This creates a proton gradient under which the external protons tend to diffuse back into the mitochondria. The structure of the membrane allows this to occur only at special sites where an ATPase is located. The passage of the proton through this structure is accompanied by the phosphorylation of ADP. 

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